1. Field of the Invention
The present invention generally relates to the manufacture of electrical devices and, more particularly, to the automated optical inspection of metal patterns on an underlying structure or substrate.
2. Description of the Prior Art
Fabrication of electrical and electronic devices which have included a layer of metal connections formed on an underlying layer or substrate has been a well-known practice. Printed circuits are a well-known example of such structures. Integrated circuits also include similar structures but of a much smaller size. Improvements in materials technology and semiconductor metallurgy, in particular, has resulted in ever greater degrees of miniaturization, circuit complexity and feature density in electrical devices which include such conductors.
As circuit complexity has increased, sophisticated electrical and optical methods of testing for defects have been developed in order to detect defects of a scale comparable to the feature size of the connector pattern formed. Automation of these techniques has become a practical necessity due to the numbers of connections usually present. These automated techniques have been generally effective in detecting actual defects in the pattern.
Electrical continuity testing is often effective and efficient in discovering defects in connection patterns since a network of signal lines may be simultaneously tested. However, there are many types of defects such as shorts to the same node and thin areas of a conductive pattern which cannot be easily detected by electrical testing. Further, the cost and complexity of some circuit modules currently being manufactured makes it economically desirable to repair defective circuits when a defect is discovered and electrical continuity testing rapidly becomes arduous if used to locate the defect for repair. For this reason, optical testing to compare a manufactured connection pattern with an intended pattern has been developed and successfully used to locate actual circuit defects. It should be noted that the location of defects may also be useful in the modification of conductor pattern designs in the manufacturing of devices which are of a scale at which repair is not, in fact, feasible. Since the optical scanning and comparison of an actual pattern with a desired pattern does not vary in procedure with the connection pattern formed, optical testing becomes economically advantageous as pattern complexity increases.
However, especially when conductors are of very small size, contamination of the surface underlying the pattern (hereinafter, simply the "substrate") can create latent defects where the conductor is successfully formed and initially exhibits electrical continuity but which, due to the topology imposed by the contamination or other metallurgical effects or dislodging of the contamination, may later break in service. Such a defect cannot be detected by electrical testing since electrical continuity is initially present. Such defects are difficult to detect optically since the resulting images are of an unpredictable size and shape and cause a wide variety of resultant pattern irregularities when the pattern is optically scanned. Such variations may be substantially smaller than the minimum feature size of the pattern being produced and potentially on a scale of acceptable manufacturing variations in a good pattern. To date, there has been no optical technique which is susceptible to automation which will reliably detect latent defects. Because of the scale of variations in optically sensed patterns relative to the scale of acceptable manufacturing variation, known optical testing techniques have either caused the rejection of devices with good patterns or failed to detect the latent defects with sufficient effectiveness to allow potential manufacturing yields to be realized and to adequately avoid the subsequent failure of components after they are put in service.
It is to be understood that optical testing equipment is commercially available to automate the testing process. Such equipment is capable of detecting variations in optically sensed patterns of a size sufficient to test patterns with any minimum feature size currently being produced and capable of detecting the shape of a variation from a desired pattern in any orientation in an automated manner. However, due to the similarity of dimension of acceptable manufacturing variation to variations in sensed patterns caused by contamination and other source of latent defects, such equipment tends to reject some good patterns while passing some containing latent defects. Both types of errors tend to increase the cost of the circuits produced and reduce the efficiency of the manufacturing process.
The illumination used in optical testing of surface patterns include so-called dark field and bright field illumination techniques used either separately or together and at different spectral frequencies in the optical testing of circuit connection patterns. Bright field illumination is very sensitive to variations in surface topology and results in many imaging artifacts representing acceptable variations in topology and which may cause difficulty in analysis of images and many false defect detections. However, latent defects due to contamination of a substrate are typically of fairly shallow topology (especially in cases where electrical continuity is maintained) and are better imaged by bright field illumination, alone.
Several different forms of image analysis are typically used in a battery of tests to determine if a connection pattern which has been produced is of acceptable quality and free from defects. However, the discovery of latent defects at low rates of false detection and undetected faults has been particularly difficult to achieve since many defects yield images similar to those produced as portions of a correctly formed desired pattern.